This investigation relates to vehicular navigation system for providing a driver with guidance in a direction to be traveled at a branch point.
A vehicular navigation system mounted in an automotive vehicle or the like provides route guidance by searching for a route from the present position of the vehicle in response to entry of a location such as the destination. In a case where the vehicle traverses a branch point such as an intersection, a route is displayed on a display screen and the direction of travel can also be instructed to the driver by a voice track, so that the driver will not need to look at the display, in order to inform the driver of whether the branch point should be passed through without turning or whether a left or right turn should be made at the branch point. For example, in a case where the heading of the approach road is taken as the reference and the angular difference between the approach heading and the departure heading at the branch point where route guidance is given is less than a predetermined angle, a decision to travel straight ahead is rendered. If the angular difference is less than the predetermined angle, the decision rendered is to turn left or right and such route guidance is provided by voice. (This technique has been proposed in Japanese Patent Application No. HEISEI 4-193572.)
However, when the direction of travel is decided by the angular difference, as in the foregoing method, a decision to travel straight ahead is always rendered and guidance to this effect given if the angular difference between the approach heading and the forward heading is very small. Consequently, it is necessary for the driver to look at a drawing of the intersection or a map displayed on the display screen in order to verify whether the direction to be traveled is erroneous or to confirm the direction to be traveled without relying solely upon the guidance given by voice.
For example, as shown in FIG. 24, assume that the departing roads with respect to an approach road at a four-way intersection are a road A 30.degree. to the left, a road B 10.degree. to the right and a road C 90.degree. to the right, and that the road C is narrower than roads A and B. If travel straight ahead is decided for all cases in which the angle is less than, say, 20.degree. with respect to the approach direction, then, when guidance is provided with respect to road B, road B is decided as being that for travel straight ahead at this four-way intersection. However, since road C is narrow, the four-way intersection is judged to be a Y-intersection which branches into the roads A and B, and there is a high possibility that the road B will be judged as being in the direction to the right. Thus, the route guidance given and the driver's judgment to do not agree. In order for the driver to confirm whether the direction to be traveled is erroneous or to verify the direction to be traveled, it is required that the driver verify the direction of travel by observing the drawing of the intersection or a map displayed on the display screen.
Further, there are many cases where the angle of intersection between the main road and exit road at the exit of a bypass or the like is very small. If the above-mentioned determination based upon direction is adopted in such case, guidance to travel straight ahead will be given for the exit road as well and it will therefore be necessary for the driver to confirm the road on the display. If the driver relies solely upon audio guidance, the driver will continue traveling as is and will pass the exit.
Further, the specification of Japanese Utility Model Laid-Open Application No. 60-13413 illustrates an arrangement in which a course in which travel is to take place next at a branch point is specified by an audio expression of eight directions (forward, backward, right, left, diagonally right, diagonally left, etc.) based upon the angle with respect to the approach direction. In accordance with route guidance based upon such audio information, guidance can be provided without having the driver observe the display and without any adverse influence upon driving.
In this navigation system, however, it is so arranged that judgment is made based upon the angle with respect to the approach direction. Therefore, when guidance is given at a branch point or intersection with a complicated shape, as shown in FIG. 25, for example, there are cases where the driver cannot accurately judge the direction in which to travel based solely upon the indication of forward direction. In other words, at a complicated branch point or intersection where one road branches into an underpass, a side road which passes along the side of the underpass and an overpass, it is difficult to specify the forward direction by the audio expression of forward, backward, right, left, diagonally right, diagonally left, etc.